This invention relates to seismometers, especially those designed to operate on the ocean floor, and more particularly to a method and apparatus for determining the angle of inclination with respect to the vertical assumed by such a unit during its operation and for leveling the seismic motion detectors carried by such a unit. Seismometers have become an integral component of geological research, especially oil and natural gas exploration. More recently a number of seismometers, commonly referred to as "ocean bottom seismometers" ("OBS's"), have been especially designed and built for remote operation on the ocean's floor in conjunction with such exploration. In such operations a seismic disturbance is artificially generated to create seismic shockwaves which pass through the earth and are refracted at interfaces of rock having diffusing densities. The refracted waves propate back to the earth's surface where they are sensed by seismic motion detectors carried in the seismometer. The use of OBS's poses certain problems not generally encountered in dry land seismometer operations. In seismic exploration on land, the persons deploying the seismic motion detector(s) used can take care to position it (them) so as to provide good seismic coupling to the earth. OBS's used today in deep water exploration are positioned either by being lowered on cables or, more generally, by being dropped in free fall from the ocean's surface. The user has minimal control over the placement of the OBS and generally has no idea of the precise nature of the surface on which the OBS has come to rest. Often, the OBS lands in a position which might not provide good coupling to the seismic waves which it is intended to record. The present invention is a simple and inexpensive method for determining the angle of inclination with respect to the vertical (hereinafter referred to simply as "the angle of inclination") assumed by a seismometer in its operating position, such as an OBS on the ocean floor, and an apparatus for preserving the orientation assumed by the seismometer for later measurement of that angle. Knowing the angle of inclination gives the user some idea of the contour of the ocean floor on which the OBS has fallen. This knowledge can also be used with other available information in later constructing the precise location of the unit on the ocean floor and in evaluating the data gathered and the causes of any failure to obtain data or suitable data. The present invention also assures good coupling of the seismic motion detector to the framework of the seismometer resting directly on the ocean bottom through which the seismic waves travel.
The present invention also comprises a method and apparatus for automatically leveling seismic motion detectors employed in a seismometer. Seismic motion detectors such as geophones are commercially available from numerous commercial sources and are in themselves beyond the scope of this invention. Generally, each such detector has a preferred "operating axis", either vertical or horizontal and will sense the component of motion occurring along an axis parallel to its operating axis. Thus, three orthogonally positioned detectors, one vertical and two horizontal, are needed to fully sense all components of seismic motion. Depending upon the nature of the geological investigation being undertaken, as few as one detector may be used. Means must be provided to align the vertically and horizontally operating detectors, where used, parallel and perpendicular to the vertical, respectively, for operation. Because of the nature of their remote operation, OBS's require self-leveling means for their seismic detectors. The use of gimbal arrangements for leveling OBS seismic motion detectors has been described by T. J. E. Francis et al., in the article "Ocean Bottom Seismograph", published in Marine Geophysical Researches 2 (1975), pp. 195-213 and by S. H. Johnson et al., in the article "A Free-Fall Direct-Recording Ocean Bottom Seismograph", published in Marine Geophysical Researches 3 (1975), pp. 103-117. Rex V. Johnson II et al., in the article "A Direct-Recording Ocean Bottom Seismometer" published in Marine Geophysical Researches 3 (1977), pp. 65-85, described the use of a "boat" floating in a liquid in a hemisphere to level seismic motion detectors in an OBS. The present invention is a novel device for leveling such detectors and, more importantly, can more easily and inexpensively be used than either gimbals or a "boat" with a clamping device such as a spring loaded plunger to preserve the orientation assumed by the leveled seismic detectors so that the angle of inclination assumed by the seismometer can subsequently be determined.